Conventional ten speed bicycle transmissions consist of a chain wrapped around a front double sprocket and a rear cluster containing five sprockets, a front and rear derailleurs for derailing the chain from one sprocket to another in response to a rider moving two control levers which are connected to the derailleurs. The derailleurs are made of a large number of delicate parts which need frequent maintenance and adjustment, and are easily damaged. The derailleur system is also hard to operate, it generates continuous noise and inefficiences in certain gears due to chain's misalignment. To reduce the misalignment thin sprockets and a narrow chain are used. This narrow chain is also designed to withstand the repeated sharp double bend that is imposed by the rear derraileur's tensioner, and it is manufactured without a connecting link which would interfere with the adjacent sprockets and the tensioner's cage. Because the popularity of the derraileur system this narrow chain is available throughout the industry and in every bicycle repair shop, and it is commercially advantageous to incorporate this chain in any new competing transmission system.
Due to the derailleur system deficiencies several attempts have been made in the past to introduce different systems, some of which are based on variable sprockets. An example is found in British Pat. No. 453712 where two flanges are stacked together with the planets being side saddled in a cantilevered position. However, such an arrangement induces severe stresses under working loads which lead to severe deformation of the variable sprocket, especially since the flanges have to be perforated and lose most of their structural integrity, and these deformations interfere with synchronization of the sprocket-segment-planet and the chain. Even with an inherently rigid design such as of the present invention, small misalignment of the variable-sprocket relative to the rear sprocket occur due to production tolerances which can be accentuated due to elastic deformation of the various components of the bicycle. This misalignment causes the chain to feed onto the variable sprocket at an angle. When the planets are in their expanded position they are spaced apart with the chain bridging the voids between them. When the chain is feeding onto the variable sprocket at an angle the chain tends to improperly engage with the planets and ride on top of the sprocket-segment-planet's teeth since unlike a standard sprocket which provides continuous guidance to the feeding chain, the voids allow the angular misalignment to be translated to lateral displacement. The problem can be corrected by sandwiching the planets between flanges which are sufficiently close to one another to also guide the chain, however, when the planets are contracted and the chain has to reach deep between the flanges, close spacing of the flanges will cause the chain to rub against one of the flanges when misalignment of the variable-sprocket and the rear sprocket occurs.
The object of the present invention is to overcome these and other problems, and to provide a simple durable system which can be economically mass produced from a combination of metal and plastic materials.